WW2 Soviet Heavy Tank Prototypes

Object 701 Prototype No.5

Soviet Union (1944-1945)
Heavy Tank – 1 Prototype Built

The first prototypes of the Object 701 showed promising results. However, they still suffered from several issues, such as reliability and armor layout and construction. With the prototypes No.5 and No.6, many of these issues had been resolved, as they were reconstructed and adapted based on the results of previous prototypes. The Prototype No.5 was one of the most well armored tanks in the Soviet Union at the time, with 200 mm of armor at the front of the turret and 140 mm in the front hull. Armed with the new 122 mm S-34-II, the tank showed promising results during trials, with few mechanical breakdowns. However, due to several external factors, such as the late stage development of the IS-3 and large strain on the ChKZ plant, the tank was never adopted into service.



The Object 701 was a direct development from the K-1 tank model presented in late 1943. By April 1944, the first prototype, named Object 701 Prototype No.0, was built. Between May and June, prototypes No.1 and No.2 were built and tested between June and July. In August, prototypes No.2 and No.3, which were specifically built for ballistic trials, were shot at with a variety of anti-tank guns, of both Soviet and German origin, and the overall superiority of the Object 701 over existing heavy tanks in respect to armor protection was noted. After the trials were concluded, a list of improvements was made and, on 11 August 1944, an order from the People’s Commissar of the Tank Industry issued that, by 1 October, the ChKZ plant was to rework and build new prototypes of the Object 701 taking into account the various improvement areas resulting from the ballistic trials.

The Object 701 Prototype No.1, on which Prototype No.5 would be based upon.
Source: Soviet Heavy Tank IS-4

Subsequently, prototypes No.1 and 2 were dismantled and analyzed and over 22 components were improved, including the clutch and gearbox, and several elements were modified, such as the cooling fans, lower hull, and final drive. On 21 September 1944, the armor improvements based on the results from the ballistic trials were ready. The upper frontal plate was to be thickened from 120 mm to 140 mm, driver’s ‘bulge’ to be made out of welded plates as opposed to cast, turret armor thickened from 160 mm to 200 mm, and turret roof strengthened to 45 mm.

By the end of September, both prototypes No.1 and No.2 were being rebuilt and the new changes were implemented. Thus, prototype No.1 would be renamed to prototype No.5 and prototype No.2 to prototype No.6, which featured a redesigned turret, which would be the basis for the IS-4’s turret.

Front view of Object 701 Prototype No.5 during trials at Kubinka, winter 1944-1945.
Source: War Thunder Forums
View of the Object 701 Prototype No.5 with its turret in the traveling position, swung over the engine deck and the gun fixed on the travel lock.
Source: Soviet Heavy Tank IS-4


Due to mechanical issues and delays during the construction phase, Prototype No.5 was still not ready for trials by 3 October 1944 and only started by 25 October. During these trials, the tank covered 190 km in three days. However, several issues were noted, including that the engine had issues starting, the radiator had water leaks, and the clutch disengagement link failed. A second phase of trials was conducted in November, driving 1,000 km, after which its engine was disassembled and assessed for wear and damage.

On 29 November, the tank was sent to the Kubinka proving grounds, at the order of Commander of the Armored and Mechanized Forces of the Red Army, Y. Fedorenko, and on 15 December, a commission was created for analyzing the trials, under leadership of Engineer-Colonel A.I. Blagonravov. Trials lasted between 17 December 1944 and 24 January 1945. During this period, the tank traveled 1,051 km, of which 405 km were on asphalt roads, 623 km on dirt roads, and 23 km over obstacles such as ditches. With the tank fully equipped with ammunition, tools and spare parts (combat ready), it was able to achieve a maximum speed of 41 km/h, surpassing both the IS-2 and IS-3, the latter being prepared to enter service. However, range proved to be unsatisfactory, with just 140 km of range on asphalt roads, extended to 257 km when using the fuel in the external fuel tanks as well.

The commission praised the tank, concluding that:

“The general layout of the new prototype of the experimental tank, with a slight increase in weight, provides reliable armor, sufficient ammunition, and a compact arrangement of all the mechanisms and assemblies in the tank.”

The Object 701 Prototype No.5 driving through deep snow during trials.
Source: Soviet Heavy Tank IS-4

The V-12 engine proved to be problematic once again, as it failed due to the supercharger breaking down. The supercharger in turn failed due to defects and imperfections from the production stage, leading to the commission suggesting better quality control checks at the factory. The engine heater ensured that the diesel engine could be started even in very cold temperatures, however, it also broke down twice during the trials. It was also noted that the oil cooler had leaked.

Surprisingly, the transmission did not have any issues, which ensured that the tank could reliably reach speeds comparable to the T-34 and that the tank was allegedly easier to drive compared to other heavy tanks.

Finally, the commission concluded positively that:

“The experimental heavy tank 701 is superior to existing heavy tanks, both domestic and foreign, in its combat and technical qualities. The heavy tank 701 has strong armor protection, powerful weapons, good maneuverability and quite sufficient reliability of its units and mechanisms. Controlling the tank is simple and easy. The tank’s performance indicators are good. The tank is simple and convenient to maintain and does not require frequent maintenance. Based on the trials results, the commission considers it necessary to recommend the 701 tank for adoption in the Red Army.”

Prototype No.5 maneuvering through obstacles at the Kubinka proving grounds.
Source: Soviet Heavy Tank IS-4

Adoption into Service

On 8 February 1945, Blagonravov, submitted to the GABTU the results of the trials, and proposed the tank to enter production with 25 to 30 units in February, 50 units in April and 100 units in May, and to also use its chassis for the production of the ISU-152 and ISU-122 SPGs. At GABTU, Commanders of the Armored and Mechanized Forces of the Red Army, generals N. Biryukov and B. Korobkov were in favor of equipping their units with the new tank. Thus, GABTU sent a request to deputy chairman of the State Defense Committee L.Beria, requesting support for adopting the tank into service. However, People’s Commissar of Tank Industry V. A. Malyshev was against the idea, arguing that the tank had not been tested enough. Consequently, ChKZ officials, including Plant Director I. M. Zaltsman, Chief Designer N.L. Dukhov, Chief Motor Designer I. Trashutin, and Chief Technologist Hunt, sent a document to Malyshev on 20 March 1945, further stating its advantages over existing tanks. They stated that the Object 701 fully satisfied GABTU’s technical and combat requirements and should enter service, while also mentioning that it used a variety of modern and capable features, such as the 800 hp V-12 engine, and had high reliability and maneuverability (according to ChKZ officials). At the end of the letter, this segment was written:

“Vycheslav Aleksandrovich, for the construction and perfecting of the 701 tank according to your instructions, we and the factory workers invested a lot of work and all our love, knowledge and experience, and, despite the great challenges and difficulties, with your help we can give the Army a good new tank in the shortest time possible.

We hope, Vycheslav Aleksandrovich, for your personal participation and assistance in resolving the issue of accepting the 701 tank into service and we consider it worthy to begin production in May”

Rear view of the Object 701 Prototype No.5.
Source: Soviet Heavy Tank IS-4

However, Zaltsman understood that the ChKZ plant would not be capable of producing the IS-2, IS-3, and the Object 701 simultaneously without facing major difficulties and decreasing overall production and even quality, a risk not worth taking during wartime conditions. On 22 February 1945, Malyshev sant a letter to Beria showcasing his point of view, noting that the trials, aside from not being sufficient, were too superficial, with majority of it being on paved or dirt roads, that the engine was only stressed to 50-55% as opposed to 70-75%, and that the cooling system, transmission and engine had still not been tested sufficiently.

The officials over at the ChKZ plant tried to state that production of both tanks simultaneously was feasible, noting that they could produce 25 IS-3s and 100 Object 701s per month, and that the Object 701 was to enter pilot production of 10 units between April and May 1945, with a further 25 tanks per month until August, 50 tanks until September, and 100 tanks by October 1945.

Yet Malyshev also considered that the IS-3, which was already to enter service, was the better choice, as it could be delivered to military units by summer 1945, while the Object 701 could only be delivered by September at the earliest. He also had issues with the general design of the tank, suggesting that the armor layout was to be made more similar to that of the IS-3, and that a pilot batch of 10 tanks was to be made between May and June 1945 and tested extensively, with the question of mass production only being available by September.

To further complicate the matter, General Korobkov, who was interested in the new tank, also sent a letter to Chairman Beria, stating that the trials were indeed normal and that the tank worked well, concluding that the Object 701 was the most advanced tank at the time.

Only by March 1945, did the Generals Korobkov and Biryukov, ChKZ director Zaltsman and Commissar Malyshev agree upon the matter of the Object 701 and all signed and sent a letter to Chairman Beria, noting that the tank would undergo further testing in heavier terrain while allowing the plant to prepare for mass production. Beria agreed, however, he decided to wait until further notice with the mass production of the tank.

A second set of trials of the tank was held between 28 January and 29 March 1945, during which the tank traveled 400 km through harsh winter, of which 122 km were during the night. An average speed of 12 to 14 km/h was achieved because of the snow, and frequent stops every 4 to 6 km had to be made to clear the snow from the periscopes. During these trials, issues with the transmission were found.

The Object 701 Prototype No.5 side view during trials.
Source: Soviet Heavy Tank IS-4

By 20 July 1945, the trials of both prototypes No.5 and No.6 were finished and the two prototypes were deemed as evidence that the tank was fit for mass production. Yet Malyshev once again denied this, claiming that the production of heavy tanks was troubled already and a new tank would not be fit. Chairman Beria was requested to accept production of 10 pilot vehicles in August, with the trade-off being that the Object 701 would gradually replace the ISU-122 from production. Malyshev answered on 7 August, suggesting that Malyshev Meet with GABTU and ChKZ officials and settle the matter. However, judging by a letter from General Korobkov to Chairman Beria, Malyshev ignored this.

After these events, the Object 701 disappeared off the radar, only to emerge in February 1946 when it was proposed, once again, that the Object 701 replace the IS-3 in production, and by the end of April 1946, the Object 701 would enter service under the name IS-4.

Prototype No.5 attempting to overcome a 17° snowy slope. However, it could not due to the tracks slipping during trials in spring 1945.
Source: Soviet Heavy Tank IS-4


The design of the Prototype No.5 was mostly identical to all the previous Object 701 prototypes. The hull was loosely based on that of the IS-2, however, some changes were made. The entire hull was elongated to fit the larger engine and final drive, thus an additional roadwheel was added. The sprocket and return rollers were redesigned, while the roadwheels and idler were the same. The front of the hull consisted of a main upper plate with two side ‘cheek’ plates, which then merged with the side hull plates. The engine was located at the rear, and above it was the cooling system, which was largely based on the Panther’s cooling system. On the left side of the hull was one of the headlights, a storage box and two external fuel tanks on the rear part of the fender, with another two tanks on the other side. One of the best ways to differentiate Prototype No.5 from previous models is the three handles on the side of the turret (with other models having more or less) and the presence of two headlights.

The turret was located in the center of the hull and was similar to the one of the IS-2. The main armament was the 122 mm S-34-II gun which, due to having the gunner’s position on the right side, had a large protruding bulge in the side to fit the sight.

Side cutout view of Prototype No.5 showing the internal components.
Source: Domestic Armored Vehicles 1945-1965


The first Object 701 prototypes featured a 4-man crew: commander, gunner, loader, and driver. Due to the use of the 122 mm S-34-II gun on most of these prototypes, the gunner and commander were seated to the right of the gun, while the loader was to the left. This was particularly uncommon on Soviet tanks of the period, which normally featured the opposite layout.

The gunner was located to the right of the gun, with the manual turret traverse to his right and gun elevation controls to his left. For vision, aside from his main gun sight, he had one MK-4 periscope facing the front and one facing the right side of the turret. The commander sat right behind the gunner, and had a rotating cupola with two MK-4 periscopes and a hatch through which both the commander and the gunner would enter and exit. To his right were the intercom controls. The loader sat on the opposite side of the commander, to the left of the gun, loading the shells in with his right hand, as well as the 7.62 mm coaxial machine gun. Lastly, the driver sat alone in the front of the hull, with two MK-4 periscopes above his hatch and with one vision slit for direct forward vision.

Diagrams showing the field of view of the different periscopes of the Object 701 Prototype No.5.
Source: Soviet Heavy Tank IS-4


The main armament was the 122 mm S-34-II gun, based on the 100 mm S-34 gun developed at TsAKB in 1944 as a replacement for the D-25T on the IS-2. The first prototype was tested on the Object 701 Prototype No.0.

Ballistically, the gun was very similar to the 122 mm A-19. Intended as an improvement over the D-25T, it had a better structural design and moreover lacked a muzzle brake, which decreased the horizontal blast, improving safety for nearby infantry. A bore fume evacuation system was also added, ejecting fumes when the rammer was opened, decreasing fumes in the fighting compartment and improving rate of fire. The horizontal gun sliding breech lock was semi-automatic. The gun had an elevation of +19° and depression of -3°.

In total, 58 rounds of 122 mm were provided, with 33 in the turret and 25 in the hull.

Left side view of the Object 701 Prototype No.5.
Source: Soviet Heavy Tank IS-4
Cutout view of the top of the turret of the Object 701 Prototype No.5. The layout was essentially identical to the previous Object 701 prototypes. Note that the gunner and commander are on the right side of the gun, very uncommon on Soviet tanks of the era.
Source: Soviet Heavy Tank IS-4
Ammunition layout of the Object 701 prototype No.5.
Source: Soviet Heavy Tank IS-4


The tank was frontally immune to German 75 and 85 mm guns, with a muzzle velocity of 1,000 m/s, 122 mm armor-piercing (AP) rounds with a muzzle velocity of 800 m/s, and 152 mm AP rounds with a muzzle velocity of 600 m/s.

The side armor could withstand 75 mm AP rounds from a distance of 1,600 m, 88 mm AP rounds from 3,790 m, and 122 mm AP from a distance of 4,400 m.

The frontal plate was 140 mm thick, angled at 61° from vertical, with the lower plate being 140 or 150 mm thick. Side armor was likely 120 mm thick. The turret armor had been increased from 160 mm on previous prototypes to 200 mm at the front, while the turret roof was also increased from 30 mm to 45 mm.

Comparison of the side armor of various Soviet heavy tanks in 1945, showing the IS-3 (top left), IS-5 (top right), Object 701 (bottom left), and IS-7 (Object 257) (bottom right).


The engine compartment, situated in the rear of the hull, was separated from the central fighting compartment by a firewall. The engine was a V-12 diesel engine, outputting 750 hp and having a maximum rpm of 2,100. It was developed in the autumn of 1943. It was based on the V-12IS engine and boosted using a supercharger from an AM-38 aviation engine. The original maximal rpm of the engine was 2,200, but this had to be decreased for increased reliability once mounted in the tank.

The planetary transmission, designed by the Bauman Moscow Technical School, featured six forward and three reverse gears with a 3-stage gearbox.

On the left side of the engine was the oil tank and a fuel tank, and on the right side, the other two fuel tanks. The four radiators and cooling fans were situated above the fuel and oil tanks and were isolated from the rest of the compartment to prevent dust from entering. This layout was largely borrowed from that used on the German Tiger and Panther tanks, although the components themselves were purposely built for the Object 701.

When mounted in Prototype No.5, the average speed achieved on paved roads was of 30.6 km/h and on dirt roads of 20.8 km/h, on par with the T-34/85, which could achieve 31.6 km/h and 20.2 km/h, respectively. Top speed was 41 km/h, though it was noted that when driving through harsh terrain, like mud and snow, the average speed would drop significantly, to between 12 and 14 km/h.

The V-12 engine was used on the Object 701 prototypes and even production IS-4.
Source: Domestic Armored Vehicles 1945-1965

Further Prototypes and IS-4

Alongside the Object 701 Prototype No.5, the Prototype No.6 was also developed and tested, though it used a completely different turret and 122 mm D-25T gun. When the Object 701 was accepted into service in March 1946, it was largely based on Prototype No.6. When preparing for production in summer and autumn 1946, the IS-4 was heavily redesigned, receiving changes in all areas.

Due to development problems and reliability issues, production of the IS-4 was constantly pushed back, and when a pilot batch of tanks was built at the end of 1946, they were not accepted by the Red Army due to significant reliability and mechanical problems. The first IS-4s to enter service with the Red Army was in April 1947, nearly a year after it was accepted for production.

The Object 701 Prototype No.6
A serial production IS-4


The Object 701 prototypes No.5 and No.6 were the most advanced prototypes of the Object 701 program in 1944 and 1945. Having several mechanical components improved and the armor thickened, they fared well in trials, which led to several GABTU and military officials to request adoption into service and mass production. However, there were those who saw it unfinished and unnecessary, considering the development of the IS-3 and, most importantly, its complexity that would have decreased production of the IS-2 at the ChKZ plant during wartime. Although the Object 701 was a strong and apparently reliable tank, the context surrounding it made it unfit for production. By early 1946, when the Soviet officials finally came about to pursue the Object 701 in the form of the IS-4, the situation had changed, causing the IS-4 to be one of the most unreliable and unsuccessful Soviet tanks of the post-war period.

Object 701 prototype 5. Illustration by Pavel ‘Carpaticus’ Alexe.

Object 701 Prototype No.5 Specifications

Dimensions (L-W-H) 6,604* – 3,260 – 2,450 mm
*excluding gun barrel
Total weight, battle-ready Around 56 tonnes
Crew 4 (commander, gunner, loader, driver)
Propulsion 750 hp turbocharged V-12 diesel at 2,100 rpm
Speed 41 km/h
Armament 122 mm S-34-II
7.62 mm DT machine gun (coaxial)
Armor UFP: 140 mm
LFP: 140 to 150 mm
Side hull: 120 mm
Rear hull; 120 mm
Deck & belly: 30 mm
Turret: 200 mm
Turret roof: 45 mm
No. Built 1


Soviet Heavy Tank IS-4 – Maxim Kolomiets

Stalin’s Superheavy tank IS-7 – Maxim Kolomiets

Domestic Armored Vehicles 1941-1945 – A. Solyakin, I. Zheltov, I. Pavlov, M. Pavlov

Domestic Armored Vehicles 1945-1965 – Ivan Pavlov, Mikhail Pavlov

IS Tank – Igor Zheltov, Alexander Sergeev, Ivan Pavlov, Mikhail Pavlov

Armor Collection No.3 1996 – Mikhail Baryatinsky

Designer N.L. Dukhov and his School – Yu.V. Rozhdestvnsky, V.N. Bondar

TiV No.1 2013 p.47-60 – M. Pavlov, I. Pavlov

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One reply on “Object 701 Prototype No.5”

First line of the Armor section, I assume it should say 88mm guns, not 85.

Great article otherwise.

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